Accuracy..

[+Snosrap]

Following a suggestion on another thread I decided to try to find out how accurate my GPSr really is:

 

I took it to one of the OS Passive Stations (see www.gps.gov.uk), Skell Gill near Pateley Bridge for those who care, and having entered the waypoint (approximated as accurately as the GPSr allowed) placed the receiver next to the bolt.

 

The GPSr indicated that the waypoint was 3 feet away. Doing a bit of maths allowing for the fact that I can only enter waypoints to within about 50 inches I worked out that my trusty Garmin was actually accurate to within about 4 and a half feet.

 

I know this proves nothing and accuracy changes continually but I was with the result.

 

Anyone else tested theirs (with or without WAAS)??

 

By the way, the "GPS Accuracy" stated by the machine was 16 feet at the time.

[+Team Ullium]

I must admit Snosrap your findings does not surprise me in the least

 

And although you (and I both) admit that it perhaps proves nothing overall....I rather think it substantiates the point I was attempting to make over on the other thread

 

However, my jury is still out on this whole subject!!

 

Ullium.

[+The Forester]

I worked out that my trusty Garmin was actually accurate to within about 4 and a half feet.

....

By the way, the "GPS Accuracy" stated by the machine was 16 feet at the time.

Thanks for your datapoint, Snosrap.

 

I'm not at all surprised to see the disconnect between the indicated "accuracy" figure and the realworld accuracy. I've noticed the same thing with my Magellan when it is operating without WAAS.

 

A couple of weeks ago I put my GPSr on a trig pillar and watched the distance figure (to the OS given co-ords) and flicked over to the page which shows the "Estimated Position Error". Repeatedly the two numbers were completely out of phase with eachother. Often while the EPE figure was up around 10 yards, the distance to the waypoint was down around 3 yards and vice versa.

 

It just goes to show that checking the actual accuracy at a known point is the best way to establish the performance of the machine in realworld conditions.

 

Where the EPE figure can be very useful is when a fix is suffering from multipath. The residual will be high, raised by whichever sats or sats are suffering the multipath. With a bit of intelligent guesswork when looking at the page which displays the azimuth/altitude of the received sats, you can manually deselect a misleading satellite line of position quite simply by blocking its raypath to the GPSr antenna with your hand. The L1 frequency which we use with these machines cannot go through water and your hand contains enough water to effectively mask the satellite's signal. If you have blocked a rogue satellite's line of position from the fix, you will see the fix's quality indicator showing a decreased number and the resultant fix will be more accurate.

 

It's one of the skills of using GPS which is easily learned and very effective at improving the actual quality of a fix.

 

Cheers, The Forester